"It's a bitter disappointment the ceremony will not take place in its traditional form."
Another insider grimly warned the Queen's absence was "very much a sign of things to come".
Buckingham Palace said the Queen's schedule was being adjusted for her comfort.
At some recent royal engagements, she has been seen using a cane to help her get around.
She usually stays at Balmoral until October, but it is understood that she will make a brief return to London - either at Buckingham Palace or at Windsor - on September 6 when Boris Johnson hands in his resignation as Prime Minister of the UK.
She will then welcome the winner of the Conservative Party's leadership contest, which will be either Foreign Secretary Liz Truss or former Chancellor Rishi Sunak.
She is then expected to return to Scotland.
The Queen was forced to pull out of Royal Ascot - one of her favourite events in the royal calendar, earlier this year.
She had attended every meet since ascending to the throne in 1952.
The monarch was replaced by Prince Charles, Camilla, the Duchess of Cornwall and Peter Phillips.
She was also absent from the Epsom Derby and a service at St Paul's Cathedral during her Platinum Jubilee celebrations, but appeared on the Buckingham Palace balcony for a fly-past.
Before that, she attended the Chelsea Flower Show with the help of a mobility scooter.
The Queen's health issues appeared to start last year, when she was forced to miss the Remembrance Day service with a sprained back.
She later came down with Covid, but continued to hold meetings via videolink.
